The invention discloses a half-wavelength alternating current transmission line fault phase selection method based on an instantaneous power sudden change, solving the problem of fault phase judgmentunder various fault types of a 1000 kV and above extra-high voltage half-wavelength alternating current transmission line. The half-wavelength alternating current transmission line fault phase selection method based on the instantaneous power sudden change comprises the following steps: obtaining an instantaneous power sudden change setting valve [Delta]p2 and an instantaneous power sudden changeratio setting value [Delta]k for fault phase selection of the extra-high voltage half-wavelength alternating current transmission line; sampling secondary voltages of A phase, B phase, and C phase ofa half-wavelength current transmission line transmitting terminal bus potential transformer to obtain three-phase voltage instantaneous values; sampling secondary currents of A phase, B phase and C phase of a current transformer of the half-wavelength current transmission line transmitting terminal at a location where protection is installed to obtain three-phase current instantaneous values; calculating three-phase power instantaneous values pa(k), pb(k) and pc(k); and comparing the sudden changes [Delta]pa2(k), [Delta]pb2(k) and [Delta]pc2(k) of 100 Hz components of the A, B and C three-phase instantaneous power with the instantaneous power sudden change setting valve [Delta]p2 separately, so that the half-wavelength alternating current transmission line fault phase can be judged quickly.